The successful implementation of a lightwave communication system requires the manufacture of high quality lightguide fibers having mechanical properties sufficient to withstand stresses to which they are subjected. Typically, the fiber has an outside diameter of 0.125 mm and is drawn from a glass preform having an outer diameter of 17 mm. Each fiber must be capable of withstanding over its entire length a maximum stress level which the fiber will encounter during installation and service. The important of fiber strength becomes apparent when one considers that a single fiber failure will result in the loss of several hundred circuits.
The failure of lightguide fibers in tension is commonly associated with surface flaws which cause stress concentrations and lower the tensile strength from that of the pristine unflawed glass. The size of the flaw determines the level of stress concentration and, hence, the failure stress. Even micron-sized surface flaws cause stress concentrations which significantly reduce the tensile strength of the fibers.
Long lengths of lightguide fibers have considerable potential strength but the strength is realized only if the fiber is protected with a layer of a coating material such as a polymer, for example, soon after it has been drawn from a preform. This coating serves to prevent airborne particles from impinging upon and adhering to the surface of the drawn fiber which would serve to weaken it. Also, the coating shields the fiber from surface abrasion, which could be inflicted by subsequent manufacturing processes and handling during installation, provides protection from corrosive environments and spaces the fibers in cable structures.
In one well known process, the coating material is applied by advancing the lightguide fiber through a reservoir in an open cup applicator containing a liquid polymer material. Typically, the fiber enters the reservoir through a free surface, and exits through a relatively small die orifice at the bottom of the reservoir. The coating material is the cured and the coated fiber taken up by a suitable capstan.
Uniform wetting of the fiber during the coating process is largely affected by the behavoir of an entrance meniscus which exists where the fiber is advanced through the free surface of the coating material in the reservoir. As is well known, the wetting characteristics of two materials such as a coating and glass, depend on the fiber temperature entering the reservoir, surface tension and chemical bonds which are developed between the two materials.
The wetting characteristics are affected by a pumping of air into the meniscus. During the coating process, both the fiber surface and the polymer surface are moving at a relatively high speed. The moving surfaces shear the surrounding air, causing it to flow into the point of the meniscus. The drawn fiber pulls a considerable amount of air into the coating material as it enters the free surface of the reservoir. Thus in the coating applicator, the entrance meniscus is drawn down with the moving fiber, instead, of rising along its surface as it does under static conditions.
It has been found that as the draw rate exceeds about 0.2 meter per second, which is less than the commonly used rate of approximately one meter per second, this pumping action causes the meniscus to extend downwardly and develop essentially into a long, thin column of air which surrounds the fiber and is confined by surface tension in the coating material. Tests have shown that the drag force is sufficiently high to sustain a column of air of considerable depth.
Air entrainment in the form of bubbles on the moving fiber occurs as relatively thin packets of air break off from the column and are carried along with the fiber on its surface. They remain on the fiber, resembling a skin, until they reach a region of pressure gradient in the vicinity of the die opening where they are compressed. This causes the air packets to bulge and form bubbled which may be removed by surrounding flow lines leading away from the fiber. Should an air packet be compressed farther downstream where all the flow lines extend out of the die with the fiber, the bubble can exit along with the fiber. As the quantity of these bubbles increases, more tend to pass through the die and remain in the coating on the fiber.
As the draw speed is increased, the meniscus becomes unstable, oscillating between a fully developed state with circulation and a relatively small one with little or no circulation. At higher speeds, the column can extend completely through the polymer coating material. In such case, the fiber no longer contacts the polymer, the meniscus collapses and the fiber undesirably exits the die with no coating material or with an intermittent, beaded coating.
One method directed to solving these problems is described in U.S. Pat. No. 4,409,263 to Aloisio, Jr. et al. wherein the fiber is advanced through a continuum of coating material, which extends from a free surface of a reservoir and through first and second dies that are arranged in tandem, at a velocity which causes air to be entrained in the coating material. A pressure gradient is established between portions of the first die adjacent to its exit orifice. The first die communicates with the reservoir and is spaced from the second die to provide a chamber which communicates with a pressurized supply of the coating material. The pressurized flow enhances the pressure gradient in the first die and establishes sufficient volumetric flow of coating material upwardly through the first die to cause bubbles in the coating material on the advancing fiber to be removed. Although such a technique is able to coat fibers at speeds of up to 9 meters per second at drawn speeds between approximately 5 to 9 meters per second, the strength of the fiber has been found to be quite low.
Various other techniques provide upper and lower sections through which the fibers are drawn wherein the lower section is pressurized by external connections and devices. Although such techniques have met with varying degrees of success, there still appears to be a need for methods and apparatus which substantially reduce, if not eliminate bubbles in the coating material in an economically efficient manner. Additionally, it is desired that the fiber be coated at a high speed (e.g., equal to or greater than 10 meters per second) while maintaining the strength of the fiber.